Lecture 14u
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Transcript of Lecture 14u
Principle of Screw andPlate Fixation
&Mechanical Behavior of
Implant Materials
Roles of Implants
• Add stability– Fracture fixation– A plate used after osteotomy
• Replace damaged or diseased part – Total joint replacement
• Healing stimulants
Advantages of Internal Fixation
• No casts– Prevent skin pressure and fracture blisters– No scars
• No complications of bed rest– Important for the elderly
• Early motion– Avoid stiffness– Enhance fracture healing– Prevent muscle atrophy
Principles of Fixation
• Rigid fixation– Stress distribution– Fracture stability
• Compression– Stability
• Primary healing– Membranous bone repair
Biomechanics of Dynamic Compression Plate (DCP)
• Designed to compress the fracture– Offset screws exert force
on specially designed holes in plate
– Force between screw and plate moves bone until screw sits properly
– Compressive forces are transmitted across the fracture
ttb.eng.wayne.edu/ ~grimm/ME518/L19F3.html
Beginning
End Result
DCP (Cont’d)
• Alternate embodiment:– External compression
screw control
• Additional pictures of internal plate
Plate Placement
• Lateral cortex
• Flexural rigidity– E * I
• Depends on direction of loading– Area moment
of inertia
Plate and oblique fracture
A: For ONLY torsional loads: 45° to long axis
B: For ONLY bending loads: Parallel to long axis
Realistically: loads in both directions will be applied: Divide angle between long axis and 45°
A
B
Dynamic Hip & Condular Screw Indications (DHS)
• Fractures of the proximal femur
– Intertrochanteric fractures
– Subtrochanteric fractures
– Basilar neck fractures
• Stable fractures
• Unstable fractures in which a stable medial buttress can be reconstructed
• Provide controlled collapse and compression of fracture fragments
http://tristan.membrane.com/aona/tech/ortho/dhs/dhs04.html
Sliding Compression Screw Devices
• Screw in center of femoral head (proximal fragment)
• Slides through barrel attached to plate– See yellow arrow
• Essential to obtain max hold capacity in head of femur
• Plate is attached to bone (distal fragment) by screws
– Screw threads designed to allow optimum fracture compression and hold
Sliding Screw Plate Angle
• 135° Plate Angle– For anatomic reduction– Less force working across sliding axis than
higher angle plates• Prevents impaction
– Used effectively in stable fractures • Controlled collapse is not important
Sliding Screw Plate Angle
• 150° Plate Angle– For unstable fractures– Mechanically, it is desirable to place sliding
device at as high angle as clinically possible while still maintaining placement of device in center of head
– Technically surgeon cannot place sliding device at high angle in small hip or in hip with varus deformity
DHS Technique
• Incisional line– Red = conventional– Green = minimal
access
• Procedure is monitored by x-ray image intensifier
http://www.maitrise-orthop.com/corpusmaitri/orthopaedic/laude_actu/laudepertroch_us.shtml
DHS Targeting Device
• Aligns guide pin
• Under the vastus lateralis
• Wedged in upper part – Between vastus
and femoral shaft
http://www.maitrise-orthop.com/corpusmaitri/orthopaedic/laude_actu/laudepertroch_us.shtml
DHS Guide Pin
• Guide pin is inserted– Centered in the
femoral neck
http://www.maitrise-orthop.com/corpusmaitri/orthopaedic/laude_actu/laudepertroch_us.shtml
DHS Axial Screw
• Axial screw is inserted with an extension– Extension to guide
the barrel of plate• Slot along screw fits
a longitudinal ridge inside barrel prevents rotation, allows axial compression only
http://www.maitrise-orthop.com/corpusmaitri/orthopaedic/laude_actu/laudepertroch_us.shtml
DHS Plate
• Plate against femoral shaft– Shaft screws are
inserted
http://www.maitrise-orthop.com/corpusmaitri/orthopaedic/laude_actu/laudepertroch_us.shtml
DHS Problems
• With the plate attached to the bone– Bone below the plate is at an increased
risk of a stress fracture
• Quality of bone is important– Procedure will vary among patients with
healthy or osteoporotic bone
Materials
http://www.me.udel.edu/~advani/research_interest/implants.htm
http://www.orthopedictechreview.com/issues/sep00/case15.htm
Composite
Ceramic
http://www.centerpulseorthopedics.com/us/patients/hip/hip_issues/index
Metal: Rough & Polished
http://www.centerpulseorthopedics.com/us/products/hip/allofit/index
Polymer
Bio Materials
• Synthetic materials – Non viable material– Interacts with biological systems
• Corrosion• Debris
– To augment or replace tissues and their functions
Types of materials
• Metals
• Composites
• Polymers– Polyethylene (PE)– Silicone
• Ceramics
• Bone cement (PMMA)
• Biodegradable
Metals TitaniumCobalt-chromium-molybdenum
Stainless Steel
Chemical Make-up
Ti6Al4V30-60% Co
20-30% Cr
7-10% Mo
Cr, Ni, Mo
Cr: oxide layer when dipped in Nitric acid (reduced corrosion)
Young’s Modulus
110 GPa 200 GPa190 GPa
(used with cement)
Benefits Yield strength; Ti > Stainless Steel
Stronger and more corrosion resistant than stainless steel; Excellent resistance to fatigue, cracking, and stress
Strong, cheap, relatively biocompatible; annealed, cold worked or cold forged; relatively ductile-contouring of plates and wires
Uses
Cementless joint replacements (total knee arthroplasty); Fracture fixation devices
Total joint arthroplasty (usually fixed with cement); Need to be inserted with a lower modulus polymer cement for fixation to prevent stress shielding of surrounding bone
Rarely used in new designs in joint replacement; Fracture fixation devices
Problems
Poor wear characteristics
*varies with smooth or porous surface
Co, Cr, Mo known to be toxic in ionic form; High modulus
*varies with smooth or porous surface
Excessively corrosive in some cases Susceptible to fatigue cracking Very high modulus PMMA cement may cause fracture or tissue reaction
http://www.engr.sjsu.edu/WofMatE/projects/srproject/srproj3.html#overview
Composites
• Manufactured in several ways– Mechanical bonding between materials (matrix and filler)– Chemical bonding– Physical (true mechanical) bonding
• Young’s modulus = 200 GPa• Benefits
– Extreme variability in properties is possible
• Problems– Matrix cracking– Debonding of fiber from matrix
• Examples: concrete, fiberglass, laminates, bone
Ceramics
• Materials resulting from ionic bonding of – A metallic ion and– A nonmetallic ion (usually oxygen)
• Benefits– Very hard, strong, and good wear characteristics– High compressive strength – Ease of fabrication
• Examples– Silicates , Metal Oxides - Al2O3, MgO – Carbides - diamond, graphite, pyrolized carbons – Ionic salts - NaCl, CsCl, ZnS
Ceramics (cont’d)
• Uses– Surface Replacement – Joint Replacement
• Problems– Very brittle & Low tensile strength
• Undergo static fatigue
– Very biocompatible– Difficult to process
• High melting point • Expensive
Polyethylene
• Ultra high molecular weight (UHMWPE)• High density
– Molecular weight 2-6 million
• Benefits– Superior wear characteristics– Low friction– Fibers included
• Improve wear properties • Reduce creep
• Used – Total joint arthoplasty
Bone Cement
• Used to fill gaps between bone and implant
• Example: total hip replacement– If implant is not exactly the right size, gaps
are filled regardless of bone quality
Bone Cement
• Polymethylmethacrylate • Mixed from powder polymer and liquid
monomer – In vacuum
• Reduce porosity • Increase strength
– Catalyst (benzoyl peroxide) may be used
• Benefits– Stable interface between metal and bone
http://www.totaljoints.info/bone_cement.htm
Bone Cement (cont’d)
• Problems– Inherently weak
• Stronger in compression than tension• Weakest in shear
– Exothermic reaction• May lead to bone necrosis
– By handling improperly or less than optimally • Weaker
– Extra care should be taken to• Keep debris out of the cement mantle (e.g., blood, fat)• Make uniform cement mantle of several mm• Minimize voids in the cement : mixing technique • Pressurize
Biodegradable materials
• Fixation of horizontal maxillary osteotomies – Totally biodegradable self-
reinforced polylactide (SRPLLA) plates
– Pins• Poly-p-dioxanone (PDS)
• Benefits– Gradual rate of absorption
• Allows an optimal transfer of support to bone as it heals
Mechanical Properties of IM
• As Implant materials have to function as bones, the mechanical properties of interest are– Elastic modulus– Ultimate tensile strength
• They are listed in order of increasing modulus or strength (in next 2 slides)
Elastic Modulus in increasing order of strength1. Cancellous bone
2. Polyethylene
3. PMMA (bone cement)
4. Cortical bone
5. Titanium alloy
6. Stainless steel
7. Cobalt-chromium alloy
Ultimate Tensile Strengthin increasing order of strength1. Cancellous bone
2. Polyethylene
3. PMMA (bone cement)
4. Cortical bone
5. Stainless steel
6. Titanium alloy
7. Cobalt-chromium alloy
Young’s Modulus
Interactive
Figure
http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/stiffness-cost/NS6Chart.html
Additional Resources
• http://www.depuyace.com/fracture_management/fracturemanage_skeltn.htm
The End
Fracture Blisters• Blisters on swollen skin overlaying a fracture
– Most often at tibia, ankle or elbow – Appear within 24-48 hours of injury
• Complicate or delay surgical treatment if present preceding care – No adverse affects if they appear following treatment
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=94046106
Varus Deformity at the kneeA. “A medial inclination of a distal bone of a joint from the midline”
– Can occur at any joint; Knee shown
B. Before correctionC. After corrective implants
http://www.wheelessonline.com/o12/74.htm http://www.hyperdictionary.com/medicalhttp://www.merckmedicus.com/pp/us/hcp/diseasemodules/osteoarthritis/diagnosis.jsp
A BC
Oteotomy
• Removal of a wedge of bone to correct a (varus) deformity– High Tibial Osteotomy
http://www.allaboutmydoc.com/surgeonweb/surgeonId.2729/clinicId.1432/theme.theme3/country.US/language.en/page.article/docId.31146
Proximal Femur (Hip) Fractures
• Risk of fracture effected by
– Age
– Gender
– Geographic location/ Ethnicity
– Mental capacity
– Bone strength
– Pre-existing medical conditions
http://www.orthoteers.co.uk/Nrujp~ij33lm/Orthhipfrac.htm